1. Field of the Invention
The present invention relates to a gallium nitride-based light emitting device and a manufacturing method for the same, and in particular to a device with fewer cracks and a method of suppressing formation of cracks.
2. Description of the Related Art
Gallium nitride (GaN)-based compound semiconductors are applied to short wavelength light emitting devices, such as LEDs.
For efficient enclosure of light in a light emitting devices of an LED, a laser, and so forth, an AlGaN layer having a thickness 0.4 μm or greater or a strained layer super lattice layer constituting of alternately stacked AlGaN and GaN is grown on a GaN layer. Because an AlGaN layer has a smaller refractive index than that of a GaN layer, sandwiching the light emitting layer by AlGaN layers encloses light from the light emitting layer within the light emitting layer.
FIG. 5 shows a structure of a general short wavelength light emitting device (370 nm to 450 nm). Specifically, an n-GaN layer 12 having a thickness 1 μm or greater is formed on a sapphire substrate 10, an n-AlGaN layer having a thickness of approximately 0.5 μm is formed as an n-clad 14 on the n-GaN layer 12, an InGaN layer is formed as an active layer 16 on the n-AlGaN layer, and an AlGaN layer is formed as a p-clad layer 18 on the InGaN layer. That is, the light emitting layer 16 made of InGaN is sandwiched by the clad layers 14 and 18. In such a structure, light from the active layer 16 is reflected by the clad layers 14 and 18, thus enclosed in the active layer 16.
However, because a lattice constant of AlGaN, the material of the clad layers, is smaller than that of GaN, lateral tensile stress is caused in the AlGaN layer as indicated by the arrows in the drawing. Larger tensile stress is caused in a thicker AlGaN layer, and tensile stress in excess of a critical value causes a crack 14a to form.
Japanese Patent Laid-open Publication No. Hei 11-68256 proposes formation of an InGaN layer and formation thereon of an AlGaN layer serving as a crack preventing layer.
FIG. 6 shows a structure of a light emitting device including an InGaN layer. A difference from the structure of FIG. 4 lies in that an InGaN crack preventing layer 13 is formed between the n-GaN layer 12 and the n-clad layer 14. The crack preventing layer 13 has a thickness in the range of between 10 nm and 0.5 μm. In the publication, it is explained that this range is selected because a thickness thinner than 10 nm will have little effect on crack prevention and a thickness larger than 0.5 μm may affect the crystals themselves.
Such a crack preventing layer 13, however, can prevent cracks only when the AlGaN clad layer has a thickness approximately 0.5 μm or less. Cracks may be caused in an AlGaN clad layer which is thicker than this value. Moreover, when InGaN is used for a crack preventing layer 13, (UV) light from the active layer 16 is absorbed by the InGaN, and light emitting efficiency is thereby deteriorated.